A typical method for programming a nonvolatile semiconductor memory cell, such as a nitride, read only memory cell (NROM), involves initially applying a programming pulse thereto, thus causing charge to become trapped in a retention layer of the cell. This trapped charge induces the threshold voltage V.sub.TH of the cell to increase.
Ordinarily, the programming pulse is followed by a program verify pulse. Via various known in the art methods, the program verify pulse verifies the programmed level of the cell. In memory cells such as the NROM, this is accomplished via a reverse read action. If the program verify pulse reveals that the cell has not yet reach the programmed level, an additional programming pulse is applied, followed by a subsequent program verify pulse. Typically, during the programming process, the programming pulses increase in voltage level, commencing at a relatively low voltage level and terminating at a higher level voltage. An example of such is described in Applicant's co-pending U.S. patent application Ser. No. 09/563,923 Programming Of Nonvolatile Memory Cells, filed on May 4, 2000 and incorporated herein by reference.
When the cell passes program verify, the cell is considered "programmed", and the programming process is terminated. If however, due to noise, charge leakage and the like, the program verify pulse of a programmed cell does not accurately verify the programmed state of the cell, further programming may induce too much charge into the retention layer, and cause a condition known as over-programming. In applications such as NROM, it is important to prevent over-programming of the cell. Over-programming of the cell creates a broad pocket of trapped charge, which reduces the longevity of the cell.
Conversely, if the cell is under-programmed, the threshold voltage V.sub.TH of the cell will not be high enough to read as programmed. Thus, inaccurate programming/verifying methods result in cell program mis-readings, and subsequent cell mis-programming.
The opposite procedure of programming is usually referred to as "erase". For electrically erasable programmable read only memory (EEPROM) cells, erasure changes the threshold voltage in the opposite direction to that of programming. It is equally important to prevent over-erasure (as over-programming), so as to avoid excessive reduction of the threshold voltage and subsequent deterioration in the quality of the cell's composition.
U.S. Pat. No. 5,523,972 discusses a programming method wherein each cell is queried for program verification. Those cells that pass verification are marked as programmed and are not queried again. Hence, since the method does not teach repeat queries, cells that have slipped below the programmed voltage threshold V.sub.TH are not discovered.
Alternatively, U.S. Pat. No. 5,172,338 discusses repeated query of the cells. Each cell that does not pass program verify, either on a previous query or on a subsequent query, receives a program pulse. However, for those cells which pass a previous program verify, yet failed a subsequent verify, it is risky to apply program pulses, since the continuance of programming subjects those cells to the possibility of over-programming.
It is thus important to devise an accurate method that supports generally precise programming of cells to a level that insures a reliability margin.